The invention relates to an apparatus for laying a plastic film on a roll, in particular for laying a polymer melt film on a chill roll.
In the production of plastic films, it is basically usual to press the melt film emerging from a slot dye onto a chill roll by means of a so-called air knife, whilst producing an adequate air velocity (dynamic pressure). In this case, the air knife extends in its longitudinal direction transversely over the width of the melt film, that is to say transversely to the take-off direction of the plastic film, air being able to emerge in the direction of the melt film via a continuous exit slot, generating a corresponding air velocity.
Since the melt film has a thicker edge, this must be pressed onto the chill roll with greater force. As a result, it is also necessary to pursue the objective of minimizing an inward step in the width of the film, that is to say to ensure that the width of the melt film remains as constant as possible during the plastic film production.
An apparatus forming the generic type has been disclosed by U.S. Pat. No. 4,255,365. The air knife disclosed by this apparatus has a multiplicity of air exit openings that are arranged in an offset manner in the longitudinal direction of the air knife and are directed onto the film web. This opens the possibility of producing different contact pressures over the length of the air knife, that is to say over the width of the plastic film web, in order ultimately to influence the thickness profile of the film.
However, in the case of this previously known apparatus, it is also necessary to use so-called "edge pipes" at the two edges of the film, in order to generate here in a deliberate way the higher contact pressure forces that are needed, matched to the respective film width. Finally, a design of this type is also only suitable for a specific width of a plastic film web.
It is therefore the object of the present invention to overcome the disadvantages according to the prior art, and to provide an apparatus improved in this regard for laying a plastic film on a roll, in particular for laying a polymer melt film on a chill roll.